Alkali metal/sulfur battery cells of a variety of configurations are now well known. In one configuration, a large number of hollow glass fiber lengths are employed as the electrolyte/membrane/divider between the (molten) alkali metal anode and the catholyte--a molten mixture of the sulfur "cathode" and alkali metal polysulfides of the type produced by the discharge reaction in the cell.
In the latter type of cells, the cathode, i.e., the cathodic current collector--generally takes the form of a spirally wrapped, metal foil--the adjacent wraps being spaced apart a distance somewhat more than the outer diameter of the fibers. The fibers are filled with the alkali metal and are vertically disposed, in parallel, between the wraps. The catholyte fills the spaces between the wraps not occupied by the fibers. The lower ends of the fibers are closed and the upper ends are open and terminate at the upper face of a "ceramic" tubesheet through which they pass in sealing engagement therewith. A reservoir space above the tubesheet contains more of the molten alkali metal, which flows into the fibers as the metal originally present in them is converted, during discharge of the cell, to cations--which migrate through the fiber walls into the catholyte. Electrons given up by the metal as it ionizes are conveyed through the unionized metal in the fibers and reservoir to an anode lead connected through an external circuit to a cathode lead. The electrons pass through the circuit (and leads) to the cathodic current collector (electron distributor), where they are taken up by sulfur to form sulfide ions in the catholyte.
The cathodic current collector can also take the form of a coiled strip of metal gauze or a large number of wire lengths vertically disposed between the fiber lengths in a "nail bed" array.
The foil, gauze or wire consist predominantly of aluminum and are coated with a thin layer of molybdenum disulfide, carbon or molybdenum metal, as disclosed in U.S. Pat. Nos. 3,749,603; 3,765,944 and 4,332,868, respectively. The presently preferred coating material is molybdenum.
Analogous Na/S cells in which the separator takes the form of a vertically disposed flat plate in which a plurality of parallel, vertical wells of capillary dimensions have been drilled out are disclosed in U.S. Pat. No. 3,915,741. The molten sodium is disposed in the wells and in a reservoir above them with which they communicate. The sulfur/sulfide catholyte is disposed around the plates in a stainless steel container and the cathodic current collector is a body of graphite fibers extending through the catholyte and electrically connected through a stainless steel pressure plate to the container.
Another type of Na/S cells in which the separator is in the form of vertically disposed hollow fibers are disclosed in U.S. Pat. No. 4,230,778. The fibers are filled with the catholyte, are externally wet with sodium and may or may not be spaced apart. Each fiber is sealed at both ends and the cathodic current is collected by a stainless steel or coated iron wire extending into each fiber through at least one of the seals. The coating on the coated wires is described only as being an "anti-corrosive layer".
U.S. Pat. No. 4,310,607 discloses a closely similar combination of closely packed hollow fibers bundled in parallel with "protectively-coated" aluminum wires extending into them as current collectors. The fibers are open at one end and are sealed together along their lengths in a manner leaving interstitial spaces between them except at their open-ended terminal portions. The latter portions pass through a tubesheet formed of the same glass as the fibers.
A variety of Na/S cells in which the anolyte/catholyte spaces are not of capillary dimensions are disclosed in the patents listed in the following tabulation. The cathodic current collecting means vary widely in configuration, internal structure or composition, as indicated in the summary descriptions given. The tabulation is organized into three main groups, according to the types of materials exposed to catholyte contact in the surfaces of the collector elements. The latter elements may or may not consist of or include container ("tank" or reservoir) walls. In the sole patent constituting a fourth "group", the collector means disclosed is not significantly different from others listed and the patent is listed only by reason of disclosing a different cell configuration.
Patents essentially redundant to others in the list, as to the collecting means, are not included in the Table.
TABLE 1 ______________________________________ CONFIGURATION/STRUCTURE/COMPOSITION OF CATHODIC CURRENT COLLECTORS IN PRIOR ART Na/S CELLS NOT OF OR CLOSELY SIMILAR TO HOLLOW FIBER TYPE Cell Group No. Patent No. Current Collector Description ______________________________________ I U.S. 3,404,035 Metal casing lined with Al or C or (1968) separate Al, C or S.S..sup.1 element connected to cathode lead. 3,883,367 Catholyte distributed in matrix of graphite (1975) yarn in contact with S.S. casing. 3,959,013 Casing of Al, steel or Fe/Ni/Co coated (1976) with Mo or C. 3,982,957 Carbon fiber matrix in contact with (1976) graphite tube containing central Al rod and intervening layer of liquid Sn or Sn/Pb alloy. 4,048,394 Porous carbon felt wrapped on central, (1977) Na-- containing, solid electrolyte tube, spaced radially from vitreous carbon- coated graphite element which may be catholyte container wall or separate element. 4,049,885 Catholyte in contact with graphite fibers (1977) twisted between aluminum wires connected to cathode lead. 4,053,689 Catholyte in matrix of carbon felt rolled (1977) up with an Mo-- or Cr-- coated Al foil which has uncoated Al tabs projecting from ends of roll for connection to cathode lead. 4,110,516 Catholyte in conductive matrix in contact (1978) with Cr layer electroplated on pre-etched Al casing wall per se or after first electroplating the Al with zinc. 4,118,545 Catholyte matrix consists of a mixture of (1978) alumina fibers with fibers consisting of or coated with carbon or graphite. In contact with conventional, conductive casing. Increased cell life claimed. Catholyte matrix consisting of graphite-coated wires which are Ni/Cr alloy or Ni/Cr alloy- coated Al or Cu. Matrix in contact with conventional, conductive casing. 4,131,226 Conventional matrix in contact with foil (1979) lining conductive container wall and consisting of 347 S.S., or Mo-- or Ni/ Cr-- coated mild steel. 4,169,120 Catholyte in pores of porous, carbonized (1979) composite of chopped carbon fibers and a cured resin; in contact with conventional, conductive casing. 4,189,531 Aluminum conductor coated with thin (1980) layer of electrically-conducting phenolic or poly(arylacetylene) resin. 4,213,933 Nickel alloy (Inconel 600, for example) (1980) with firmly adhered coating of pyrolytic carbon. 4,232,098 Ferrocarbon metal substrate coated with (1980) Fe/C/Cr alloy of duplex layer structure; inner layer &lt;50% Cr, outer layer &gt;60% Cr. 4,239,837 Titanium or graphite foil glued to interior (1980) surface of metal casing wall with a conductive adhesive. 4,226,712 A S.S. or Ni/Cr foil liner for a metallic (1981) casing, the foil being bonded to the casing and to an outer layer of at least 60% Cr, by diffused Cr. 4,287,664 Like 4,131,226 but with at least two full (1981) wraps of the liner. II 4,048,390 A ferrous metal substrate (Fe/Ni/Co, (1977) typically) aluminided by treatment with Al.degree., NH.sub.4 F and Al.sub.2 O.sub.3 remains conductive even though the aluminide reacts with the catholyte. 4,226,922 Substrate metal such as low carbon steel (1980) is diffusion coated with iron boride. Boron sulfide or carbide provided in close proximity to boronized surface to plug pinholes and makeup for continuing diffusion of boron into substrate. 4,278,708 A substrate metal having at least a surface (1981) layer of a transition series metal is coated with silicon carbide particles and heated (1000-1300.degree. C.) until a layer of a tertiary compound of Si, SiC and the transition metal is formed by diffusion. 4,279,974 The catholyte (preferably including (1981) powdered C) is in contact with a Honeycomb conductive element consisting of TiN (or electrode of S.S. or fibrous carbon) and connected configuration to the cathode lead of the cell. III. 3,966,492 A metal container wall or a separate ele- (1976) ment connected to the cathode lead is con- nected to a graphite felt matrix in which the catholyte is disposed. The portion of the matrix nearest the solid electrolyte/- separator is coated with an oxide or sulfide of a group I, II or III metal, a transition metal or Sn, Pb, Sb and Bi. 3,985,575 Different cathodic current collector/ (1976) distributors ("electrodes") used for charge and discharge. Discharge electrode, preferentially wet by sulfur, is graphite. The charge electrode, preferentially wet by sodium polysulfates, is a porous conductor consisting of: (a) a substrate metal, such as S.S., coated with an oxide or sulfide of a Group I, II, III or transition group metal or Sn, Pb, Sb and Br; (b) surface oxidized graphite (i.e., graphite coated with graphite oxide); or (c), electrically-conductive intercalote compounds of graphite, such as graphite bromide. 4,117,209 The catholyte matrix is a graphite felt (1978) connected to an aluminum element coated with an intermediate layer of Ni/Cr alloy and an outer layer of TiO.sub.2 or other electronically conductive oxides. British A binary Ni/Cr alloy or a ternary 2,013-022 Ni/Fe/Cr alloy is coated with a (1979) predominantly NiO film. 4,173,686 An aluminum element is coated with (1979) aluminum oxide, except where penetrated by welds making metal to metal contact with a surrounding metal element having a Ni/Cr surface layer. 4,248,943 The metallic current collector surfaces (1981) which would otherwise contact the catholyte are coated with a chromium/ chromium oxide layer. IV. 4,226,923 Conventional cathodic current collecting (1980) means in a cell configuration which minimizes amount of sodium on one side of electrolyte (separates and maximizes access to other side by polysulfide component of catholyte during charging). ______________________________________
Among the foregoing patents, those in group III are the most relevant to the present invention--which, to anticipate, is based on the discovery that the lifetimes of Na/S cells in which the cathodic current collector is a molybdenum-coated, aluminum foil are increased by baking the foil in air. The most pertinent patents in group III are U.S. Pat. Nos. 3,985,575 and 4,117,209.
U.S. Pat. No. 3,985,575 teaches that metal substrates coated with metal oxides or sulfides are preferentially wetted by the polysulfide component of the sulfur/polysulfide catholyte--which is essential for the cathodic current collector (distributor, more accurately) to be used during charging of the cell. It is also essential, of course, that the surface layer consist, at least predominantly, of electronically-conductive oxides or sulfides.
The specific combination of an aluminum substrate, a molybdenum overlayer and a surface layer of a metal oxide is not disclosed in the patent. Although MoS.sub.2 is specifically mentioned as a suitable surface layer, and MoO.sub.2 is known to be a conductive oxide, the use of MoO.sub.2 in the surface layer does not appear to be contemplated.
U.S. Pat. No. 4,117,209 teaches the combination of an aluminum substrate, a chrome/nickel intermediate layer and a surface coating of a conductive metal oxide--most notably, TiO.sub.2. Molybdenum oxides are not included in the list of other suitable oxides.
Neither of the foregoing patents suggests any effect of a metal oxide coating on cell life.
Although directed to production of a surface layer of a cathode-reactive oxide (MoO.sub.2) on a conductive metal substrate (such as Al), rather than to a current collector per se, two patents--U.S. Pat. Nos. 4,245,017 and 4,281,048, not included in the preceding list--are relevant to the present invention as embodied in a coated foil. The MoO.sub.2 layer disclosed in these patents functions as the sacrificial cathode in an Li/MoO.sub.2 cell and the substrate metal functions as a cathodic current collector therein.
The MoO.sub.2 is derived from a precursor coating of MoS.sub.2 particles, by oxidizing the MoS.sub.2 to MoO.sub.2, or to MoO.sub.3 --which is then reduced to MoO.sub.2. Since the oxidations must be carried out at temperatures below the melting point of the substrate metal, the oxidation temperature is far below the sintering temperature for molybdenum oxides. Thus, the MoO.sub.2 product of either operation (or the intermediate MoO.sub.3 layer) necessarily is obtained in the form of discrete particles. The MoS.sub.2 particles are applied to the substrate as a suspension in oil. Most of the oil is removed in the baking (oxidation) process but carbonization also results and the MoO.sub.2 (or MoO.sub.3) particles apparently are bonded to the substrate metal (and each other, presumably) by the carbonization product.
The present applicants know of no prior art more relevant to their invention than the several patents referenced herein which disclose Mo/Al or MoS.sub.2 /Al for cathodic current collection in Na/S cells or the intermediate MoO.sub.3 layer formed in one method of preparing cathode-reactive MoO.sub.2 particles on an aluminum current collector for Li/MoO.sub.2 cells. Thus, the latter art does not appear to contemplate Al/Mo/MoO.sub.3 or MoO.sub.2 structures having utility as cathodic current collectors in alkali metal/sulfur cells. Neither does said art suggest that the useful lifetime of an alkali metal/sulfur cell in which molybdenum-coated aluminum is used for cathodic current collection can be extended by formation of more than a superficial layer of molybdenum oxides on the exposed surface of the molybdenum.